Update api-ref for enhanced tag mechanism

Tag mechanism has two extensions. They supports networks,
subnets, ports, subnet pools, and routers.
This patch describes which extensions supports them.

Change-Id: I0eff6f69ab71066fc9a5c4ea35d6d9af11e37571
Related-Bug: #1662102

api-ref/source/v2/tags.inc

@@ -3,13 +3,24 @@
 Tag extension (tags)
 ====================
 
-Shows details for, updates, and deletes tags. This extension is
-available since Mitaka release.
+Shows details for, updates, and deletes tags.
 
 The maximum number of characters allowed in a tag is 60. If the length
 is longer than 60, the API returns the HTTP ``Bad Request (400)`` response
 code with 'invalid input for operation' error message.
 
+Tag Extension
+=============
+
+The ``tag`` extension allows users to set tags on their networks.
+This extension supports networks only.
+
+Enhanced Tag Extension
+======================
+
+The ``tag-ext`` extension allows users to set tags on their resources.
+This extension supports subnets, ports, routers, and subnet pools.
+
 Replace all tags
 ================